The high-precision measurement of surfaces with the aid of white-light interferometry is a standard measurement method, for example, in diesel injection technology for testing surfaces of components.
Seminal explanations of white-light interferometry are presented in T. Dresel, G. Hausler, H. Venzke, “Three-dimensional sensing of rough surfaces by coherence radar”, Applied Optics Vol. 31, 919, 1992 and in P. de Groot & L. Deck, Journal of Modern Optics, “Surface profiling by analysis of white-light interferograms in the spatial frequency domain”, Journal of Modern Optics, Vol. 42 389-501, 1995.
Furthermore, white-light interferometry may be used for determining the thickness of layers that are transparent to the radiation used.
German patent document DE 10 2006 016 131 A1 discusses an interferometric measuring device for measuring layer thicknesses of semi-transparent layers on substrates using a scanning device which scans these layers automatically in the direction of their depth (Z), with the aid of which an interference plane (IE) is displaceable relative to the layer structure, using an interferometer part (IT) having a white-light interferometer (WLI) and/or a wavelength-scanning interferometer (WLSI), to which an input radiation is supplied by an irradiation unit (LQ), the radiation being split by a beam splitter (ST) and supplied, on the one hand, to a reference arm (RA) via a reference beam path as a reference beam (RST) and, on the other hand, to an object arm (OA) having the layer structure at the time of the measurement via an object beam path as object beam (OST), having an image recorder (BA), which records the interfering radiation returned from the reference arm (RA) and the object arm (OA) and converts it into electrical signals, and having a downstream analyzer device (AW) for providing measurement results.
It is provided that the scanning device is designed in such a way that, with a constant reference beam path and object beam path, the corresponding scanning path is at least as long as the distance to be expected or ascertained in a pre-measurement between at least two consecutive boundary surfaces of the layer structure to be detected, optionally including an expected depth structure of the boundary surfaces to be expected, and that                a) In designing the interferometer part (IT) having the irradiation unit (LQ) as white-light interferometer (WLI), the coherence length (LC) of the input radiation is selected to be at maximum such that the interference maxima of the correlograms occurring consecutively during the depth scanning are distinguishable on the boundary surfaces to be detected, and/or        b) In designing the interferometer part (IT) having the irradiation unit as a wavelength-scanning interferometer (WLSI), the irradiation unit (LQ) is designed for narrow-band, tunable input radiation, the bandwidth of the input radiation being selected such that the smallest distance to be expected or estimated via the pre-measurement of the consecutive boundary surfaces to be detected is resolvable, and/or        c) In designing the interferometer part (IT) as a wavelength-scanning interferometer (WLSI) having a spectrally broad-band irradiation unit and a wavelength-scanning optical spectrum analyzer as a detector, the bandwidth of the input radiation is selected such that the smallest distance to be expected or estimated via the pre-measurement of the consecutive boundary surfaces to be detected is resolvable, and        d) The wavelength spectrum used of the irradiation unit (LQ) is adapted to the spectral transparency of the layer to be measured in such a way that this layer is at least partially transparent to the radiation.        
German patent document DE 10 2006 016 131 A1 furthermore discusses a method for interferometric measurement of layer thicknesses of semi-transparent layers on substrates, in which an interference plane (IE), which is determined by the optical path length of an object beam (OST) guided in an object beam path and by the optical path length of a reference beam (RST) guided in a reference beam path, is displaced relative to the position of the layer for depth scanning of the layer structure in the direction of depth (Z), and an interference pattern is generated using methods of white-light interferometry or of wavelength-scanning interferometry, and the interference pattern is recorded with the aid of an image recorder (BA) and automatically analyzed with the aid of an analyzer device (AW) in order to display the measurement results relating to the boundary surfaces of the layer structure.
It is provided that in depth scanning of the layer to be measured and of the boundary surfaces delimiting it, the object beam (OST) is guided over the same object beam path in a scanning cycle, and the reference beam (RST) is guided over the same reference beam path, and that, when using the method of white-light interferometry, the coherence length (LC) of the input radiation of an irradiation unit (LQ) injected into the interferometer is selected to be at maximum such that the interference maxima of the correlograms (KG)occurring consecutively on the boundary surfaces to be detected during depth scanning may be distinguished and, in using the method of wavelength-scanning interferometry, the bandwidth of the input radiation is selected such that the smallest distance to be expected or estimated by pre-measurement of the boundary surfaces to be detected is resolved, a wavelength spectrum of the irradiation unit (LQ) being selected for which the layer to be measured is at least partially transparent to the radiation.
In the interferometric measuring device and for carrying out the method, double correlograms of the radiation reflected by the layer surface and the layer back face must be analyzed. The disadvantage here is that only layers greater than approximately 2 μm may be measured, since otherwise the superimposition of the correlograms is no longer separable.
It is furthermore disadvantageous that only the thicknesses of layers transparent or at least semi-transparent to the light used or the electromagnetic radiation used may be measured. For measuring the layer thickness of carbon-based wear-protection layers, known as C layers, the document DE 10 2006 016 131 A1 therefore proposes that the wavelength spectrum of irradiation unit LQ be in the range of 1100 nm to 1800 nm. In this wavelength range, the C layers are semi-transparent due to their optical characteristics; therefore, on the top (air/C layer boundary surface) and on the bottom of the layer (C layer/substrate boundary surface) a detectable correlogram may be obtained. The limitation here also is that only layers having a thickness greater than approximately 2 μm may be measured, since otherwise the superimposition of the correlograms is no longer separable. It is furthermore necessary that the layer reflects a sufficient portion of the radiation on the surface and lets through a sufficient portion, which requires accurate adaptation of the wavelengths used and of the measuring device to the layer characteristics.